Using a Galaxy-Sized Telescope to Rethink Supermassive Black Hole Evolution
Vikram Ravi, Swinburne University of Technology
Galaxies, with their incredible diversity of shapes and sizes, are the basic building blocks of the visible Universe. The present cosmological paradigm, which explains essentially all cosmological-scale observations, posits that larger galaxies were assembled through the gravitationally driven mergers of smaller galaxies.
A supermassive black hole is thought to lie at the centre of all galaxies at least as large as our Milky Way. Such black holes, some with masses up to 1011 times the mass of the Sun, are understood to be crucial to the formation of massive galaxies. For example, the radiation from gas heated as it falls onto supermassive black holes can quench the production of new stars in a galaxy, hence regulating its mass. However, the compact, `black’ nature of black holes makes their demographics and evolutionary mechanisms difficult to ascertain.
Mergers of massive galaxies are predicted to host pairs of orbiting supermassive black holes. These pairs are driven to coalesce by the emission of gravitational waves: travelling distortions in the structure of space predicted by Einstein's general relativity that are yet to be detected. These gravitational waves, which have wave-periods of years, affect the observed rotations of remarkable stars in our Galaxy called pulsars. Pulsars are the collapsed neutronstar remnants of supernovae, and can rotate up to once every millisecond. They emit beams of radio waves that are observed as discrete pulses during each rotation, as the beams sweep by the Earth. For some pulsars, large radio telescopes can be used to time-tag the arrival times of the pulses to accuracies better than 30 nanoseconds.
For the last decade, an international collaboration of astronomers has been using the Parkes radio telescope in Australia to search for signatures of gravitational waves in data on 20 pulsars in our Galaxy. Models for the numbers of black hole pairs suggest that evidence for gravitational waves should now be present in the data. However, no gravitational waves have been detected.
This startling result necessitates a shift in thinking on the fates of supermassive black holes in galaxy mergers, or indeed on the ubiquity of supermassive black holes in galaxies. For example, it may be that galactic-centre environments are less dense than thought. This would mean that supermassive black holes in galaxy mergers never form gravitational waveemitting pairs. In this case, numerous black holes may wander the outskirts of massive galaxies. Alternatively, contrary to current thought, not every galaxy may host a central supermassive black hole, hence reducing the numbers of black hole pairs and the gravitational wave signal.
In my talk, I will introduce the concepts behind the gravitational wave detection project. I will further describe both the result and its implications for the understanding of supermassive black hole and galaxy assembly.